This invention relates to an electronic ballast with a protection circuit for small-diameter gas discharge lamps and for compact fluorescent lamps in particular. The protection circuit acts to shut down an electronic ballast when a fault condition is sensed.
Fluorescent lamps contain a tungsten filament in each end of the lamp which is coated with an emissive material that has a lower work function than tungsten. As fluorescent lamps age, the emissive coating material on the filaments is worn away so that the arc must flow from the bare tungsten filament. The work function of the tungsten filament is high, so several watts of power are dissipated in the cathode fall region near the filament. This extra power dissipation can lead to overheating of the lamp. In some cases, the glass tube may melt or crack. Occasionally, the plastic base of compact fluorescent lamps may become deformed.
When a lamp with a worn-out filament is operating with a high cathode fall voltage, the arc voltage increases. The arc voltage may become asymmetrical if one filament is more worn than the other. The increased voltage due to one or more worn-out filaments can be sensed and used to actuate a shutdown circuit.
Prior attempts at gas discharge lamp ballasts with shutdown circuits have resulted in implementations with various drawbacks. U.S. patents RE 32,901, RE 32,953, and 5,004,955. show ballast shutdown circuits that sense an overvoltage condition in the resonant output circuit. They require a voltage clamp (varistor) to conduct before the shutdown circuit can be activated. The purpose of these circuits is to prevent a shock hazard at the output terminals and not to protect the lamp. They are designed to sense large output voltage levels that occur when a lamp does not strike. Consequently, they may not be able to sense the arc voltage levels associated with overheating, which are not nearly as large.
Other prior art shutdown circuits that were designed to sense large overvoltage conditions could be adjusted to trigger at lower voltage levels because their sensing circuits do not clamp the open-circuit voltage. These circuits, however, typically use a diac as the threshold sensing device. Typical diacs have a loose tolerance on the trigger voltage level, and therefore may not have the accuracy required for sensing overvoltage levels associated with lamp overheating. Examples of non-clamping, diac triggered circuits are found in U.S. Pat. Nos. 4,398,126, 4,503,363, 4,667,131, and 5,436,529. These circuits may also have other disadvantages. U.S. Pat. No. 4,398,126 uses a costly high-voltage silicon controlled rectifier (SCR), and requires turning the power off and on after a new lamp is installed before the lamps will strike. The need to toggle the input power is not a desirable feature when many ballasts are operated on one AC circuit. U.S. Pat. Nos. 4,667,131, and 4,503,363 show protection circuits that have complicated shutdown circuits. A sense voltage related to the arc voltage triggers a diac which triggers an SCR. The SCR turns on a transistor which shuts down the inverter. The shutdown transistor is required in this circuit because the conduction voltage of an SCR is greater than the turn-on voltage of a bipolar transistor. U.S. Pat. No. 5,493,180 shows circuits that are capable of sensing lamp voltages associated with overheating, but they are fairly complicated.
U.S. Pat. Nos. 4,562,383 and 5,436,529 show shutdown circuits that have the desirable property of causing the ballast to remain off until the bad lamp is replaced. These circuits, however, suffer from other problems described above.